1. Field of the Invention
The present invention relates to a method for manufacturing liquid crystal alignment film. The present invention also relates to a liquid crystal alignment film obtained by the manufacturing method concerned, and an optical film comprising at least one of the liquid crystal alignment films concerned. The liquid crystal alignment film of the present invention may be used as optical films, such as retardation plates, viewing angle compensating films, optical compensating film, and elliptically polarizing film, independently, or in combination with other films. Furthermore, the present invention relates to an image viewing display, such as liquid crystal displays, organic electro luminescent displays, and PDPs, comprising the optical films.
2. Description of the Related Art
Recently, in optical fields, optical electronics fields etc., retardation plates for controlling a phase of light is one of important optical elements. In addition, elliptically polarizing plates can be obtained by laminating retardation plates to polarizing plates, and various configurations have been discussed in relation to retardation values, wavelength dispersion etc. of the retardation plates.
Conventionally, retardation plates have been manufactured by uniaxial stretching or biaxial stretching of polymer films. For example, polycarbonate resins may be mentioned as materials of polymer films, but the resins have large wavelength dispersion and give higher retardation in shorter wavelength side. Alternatively, there have been used retardation plates using norbornene based resins having smaller wavelength dispersion, and modified polycarbonate resins having higher retardation in longer wavelength side. These retardation plates are used by lamination with various axial angles. However, as compared with polycarbonate resins, norbornene based resins and modified polycarbonates give higher costs. The materials laminated therewith result in still higher costs. It is necessary for these laminated materials to be punched with various degrees of axial angle and to be used in a form of a single plate for laminate, therefore higher process costs and period to be need, leading to cause deterioration.
On the other hand, as retardation plates, liquid crystal alignment layers formed on alignment substrates have been known. The liquid crystal alignment layers are obtained by a method coating solution crystal materials, such as liquid crystal monomers or liquid crystal polymers, on alignment substrates and aligning uniformly, and then curing (Refer to Japanese Patent No. 2784680 specification). Liquid crystal materials include rod-like nematic liquid crystals, disk-like discotic liquid crystals, etc., and also include various kinds according to wavelength dispersion characteristics as in retardation plates using stretched films. As to substrates for alignment, stretched polymer films and rubbing alignment layers may be mentioned.
In the case of aligning a liquid crystal material on a stretched polymer film, precision in an axial direction of the stretched polymer film are necessary. However, in stretched polymer films manufactured by conventional method of longitudinal stretching and transverse stretching, it is hard to cheaply and stably obtain precision in an axial direction of not less than ±1 degrees in a width direction. For this reason, stretched polymer films may be used as alignment substrates of C-plates that satisfies “nz>nx≈ny” or “nx≈ny>nz”, where refractive indexes in a plane of C-plate are defined as nx and ny, and a refractive index in a thickness direction is defined as nz, and cholesteric liquid crystals, without necessity of precision in an axial direction. However the stretched polymer films have difficulty in use for manufacturing of A-plates that satisfies “nx<ny≈nz” or “nx>ny≈nz”, with necessity of precision in an axial direction. On surfaces of rubbing alignment layers, contamination given before coating of liquid crystal materials reduced alignment of liquid crystal materials, and thus was difficult to obtain liquid crystal alignment layer with excellent alignment. For example, alignment of liquid crystal materials will be reduced by contact of rubbing alignment layer surfaces to rolls etc. before coating process of liquid crystal materials after rubbing treatment of substrate films. Therefore, methods for manufacturing liquid crystal alignment films using rubbing alignment layers have had difficulty in introduction of continuous production with a roll to roll system.